The turnover of recombinant tissue plasminogen activator (t-PA) was studied in the presence or absence of t-Pa I peptide (7-20). This peptide is derived from he finger region of the t-PA molecule and was shown to inhibit the binding of t-PA to human umbilical vein 125I whereas recombinant t-PA was labelled with 131I. In each experiment 125I-labelled peptide was injected concomitantly with a 131I-labelled t- PA intravenously into each of six rabbits. In three rabbits the same material was injected in the presence of unlabelled t-PA peptide. Blood samples were obtained from the femoral vein and the radioactivity of both labels was quantitated. The disappearance of both labels was then plotted and halflife data was calculated using the computer program RS-1 and the equation: CoX = Ce-ax + ce-bx The results demonstrated a larger area under the curve (AUC) for t-PA in the presence of unlabelled t-PA I peptide whereas the clearance of the t-PA I peptide was unaffected. These results indicate that the unlabelled peptide may compete for a t-PA binding site on endothelial cells and alter the removal of exogenously added t-PA from the circulation. In addition the clearance of 3H-fibrinopeptide was evaluated in rabbits. The results demonstrated a dose related proportional clearance of fibrinopeptide by the kidneys and the liver respectively.